Dendritic spines are the major sites of synaptic input in the mammalian CNS and have been traditionally been considered stable structures. Nevertheless, as initially suggested by Francis Crick and confirmed recently by data from our group and by others, spines are motile in both dissociated cultures and in brain slices. Spine motility is action-based and appears to be intrinsic to the neuron. Because of the importance of spines in the cortical circuit, spine motility could have potentially, major consequences in the development and function of the cortex. In our previous work we discovered that spine motility in mouse cortex is down-regulated during the postnatal ages that herald the end of the critical period for monocular deprivation. Although the critical period in primary visual cortex has been studied extensively for many decades, it is still unclear what factors terminate it. Based on this correlation we hypothesized that the end of the critical period is due to the lack of motility of the spines. We want to examine this hypothesis in detail combining gene-gun GFP transfection, two-photon imaging, image deconvolution and electron microscopy of spines in brain slices from mouse primary visual cortex, as well as in vivo imaging, deprivation and pharmacological experiments. The first aim will focus in characterizing the motility in different cortical layers in mouser V1B and in reconstructing at the ultrastructural level the previously imaged spines. The ability of finding in serial reconstructions the same spines imaged in two-photon time-lapse movies will allow us to examine with unprecedented detail whether there are any correlations between the presence and type of motility and the presence and type of presynaptic terminal. The second aim will seek to identify the cellular mechanisms mediating the motility, with special emphasis on the downstream targets of the Rho family of small TGPases and in the examination of the role of synaptic activity in this process. The third aim will directly test if spine motility lays a causal role in the critical period, by examining whether it exists in vivo and by analyzing the consequences of blocking it in the monocular deprivation paradigm. These studies will shed light on the role of structural plasticity in the development of the visual cortex. In addition, they will help discern the cortical consequences of monocular deprivation, effects which may underlie amblyopia and strabismus, as well as help design therapeutic strategies aimed at compensating for these deficits. A more complete understanding of the development of visual cortex will also improve the measurement of acuity, contrast sensitivity and chromatic sensitivity of preverbal children and in early diagnosis of visual pathologies.